The present invention relates to a liquid crystal display and, more particularly, a liquid crystal display having repair lines and methods of repairing a defect thereof.
The liquid crystal display (LCD) is a device comprising a thin film transistor substrate, a color filter substrate, and liquid crystals inserted between the two, whereby the electro-optical effects of the inserted liquid crystals are used for display purposes.
The display method above may use simple matrix method or active matrix method. For LCDs using the active matrix method, thin film transistors (TFT) are generally used as switching devices for controlling operation of each pixel, and the TFT substrate of such LCDs comprises TFTs, pixel electrodes, gate lines for applying signals to pixels aligned in matrix formation, and data lines for applying picture signals.
When a data line of the active matrix LCD is disconnected or defective (hereinafter "disconnection" will be used to refer to either a disconnection or defect in a data line), the disconnection can be repaired by using lines within the unit pixel or using repair lines placed outside of the substrate of LCD. Under the former method, one must first locate the disconnected data line and then find the point of disconnection on the data line. However, it is difficult to find the point of disconnection and the repair becomes impossible when the disconnection occurs over two or more pixels.
Under the latter method, it is only required to locate the disconnected data line and there is no need to find the point of disconnection. Then only the crossing points of the disconnected data line and the repair line has to be shorted by laser, allowing for a much simpler repairing method.
From this point, the wiring structure of TFT substrate of conventional LCD and its disconnection repairing method shall be explained in detail. FIG. 1 is a wiring structure of conventional TFT substrate having two repair lines in the shape of a closed loop. As shown, a plurality of gate lines (G.sub.l.about.G.sub.n) are arranged in horizontal direction, and a plurality of data lines (D.sub.1.about.D.sub.m) are arranged in vertical direction, and each of gate lines and data lines has an input pad on one end. A pixel is defined by the gate line and the data line. An active area consists of all the pixels.
On the outside of the active area 50 having (n x m) number of pixels, two repair lines 100, 200 form two closed loop, each overlapping all data lines and gate lines at two points.
FIG. 2 illustrates a repairing method of the TFT substrate shown in FIG. 1. Suppose data line D.sub.4 and data line D.sub.7 are disconnected at point a and point b, respectively. The data line D.sub.4 may be repaired by shorting its two crossing points a.sub.1, a.sub.2 with the repair line 100 and disconnecting two points marked//, of the repair line 100 with laser so that picture signals could flow through the shorter path. In this way, a picture signal is sent to the disconnected point a of the data line D.sub.4 by path P.sub.1 from the top, and the signal is sent to the remaining portion of the data line D.sub.4 by path P.sub.1 '.
Then the data line D.sub.7 may be repaired by shorting its two crossing points b.sub.1, b.sub.2 with the repair line 200 and disconnecting two points of the data line D.sub.7 marked by //, of the repair line 200 with laser. In this way picture signals as with before are sent to the disconnected point b from top by path P.sub.2, and the signal is sent to the remaining portion of the data line D.sub.7 below the disconnection by path P.sub.2 '.
However, the TFT substrate having repair lines outside of the active area in closed loops would experience substantial electrical resistance when repairing a disconnected data line in the middle of active area since the signal path for sending signals to the data line below the disconnection becomes quite lengthy. In addition to large resistance, there would be a substantial parasitic capacitance increase since the repair line overlaps numerous data and gate lines, resulting in increase of RC delay and signal distortion.
Therefore, the conventional repair line structure above is not suitable for a large LCD with many data lines and gate lines. It is also inconvenient since it involves an additional step of disconnecting the repair line to guide the flow of signals through the shorter path around the repair line, as well as a step of shorting the crossing points of the repair line and the disconnected data line. Moreover, the most data lines that could be repaired are limited to four.